Heretofore, various proposals have been made with respect to catalysts for gas phase catalytic oxidation of an unsaturated aldehyde such as acrolein or methacrolein with molecular oxygen to produce an unsaturated carboxylic acid such as acrylic acid or methacrylic acid. These catalysts are required to have a high conversion of the unsaturated aldehyde or a high selectivity for the unsaturated carboxylic acid as the desired product, as far as possible, from the viewpoint of the effective utilization of the unsaturated aldehyde material to be produced from an olefin and streamlining of the process steps in the reaction. In such a case, for example, the reaction to produce acrylic acid from acrolein is carried out usually in a scale of 3,000,000 tons/year, and if the above conversion or selectivity is improved by 0.1%, the amount of acrylic acid as the product to be obtained, will be remarkably increased at a level of a few hundreds to a few thousands tons. Accordingly, improvement of the catalyst performance such as the conversion or selectivity, even if it is a little improvement, will substantially contribute to the effective utilization of the resource or streamlining of the process steps.
Heretofore, various proposals have been made with an aim to improve the catalytic performance such as the starting material conversion or selectivity in the reaction, and various proposals have been made also with respect to an antimony-supplying source compound.
For example, Patent Document 1 discloses a method for producing a Mo—V type oxide catalyst represented by MOaVbWcCudXeYfZgOh (wherein X is at least one element selected from Sb and Sn, Y is at least one element selected from Mg, Ca, Sr and Ba, Z is at least one element selected from Ti, Zr and Ce, and O represents oxygen, a, b, c, d, e, f, g and h represent atomic ratios of the respective elements, when a=12, 2≦b≦14, 0≦c≦12, 0<d≦6, 0<e≦5, 0≦f≦3 and 0≦g≦10, and h takes a numerical value determined by the oxidized states of the respective elements), wherein at least one type of antimony oxides wherein the valency of antimony is larger than 0 and less than 5, is used as at least a part of the antimony-supplying source.
Further, Patent Document 2 discloses a method for preparing a catalyst for the production of an unsaturated carboxylic acid, which contains at least phosphorus, molybdenum and antimony in the catalyst composition, characterized in that antimony trioxide having an average particle diameter of at most 0.2 μm, is used as the starting material.
Further, Patent Document 3 discloses a catalyst which has a composition represented by Mo12VaWbCucSbdXeYfZgOh (wherein X is at least one element selected from the group consisting of an alkali metal and thalium, Y is at least one element selected from the group consisting of Mg, Ca, Sr, Ba and Zn, Z is at least one element selected from the group consisting of Nb, Ce, Sn, Cr, Mn, Fe, Co, Sm, Ge, Ti and As, a, b, c, d, e, f, g and h represent atomic ratios of the respective elements, against 12 of Mo atom, 0<a≦10, 0≦b≦10, 0<c≦6, 0<d≦10, 0≦e≦0.5, 0≦f≦1 and 0≦g<6, and h is the number of oxygen atoms required to satisfy the valencies of the above respective components) and which is prepared by using antimony acetate as an antimony material source at the time of preparation of the catalyst active component.
These conventional composite oxide catalysts exhibit excellent properties respectively, but a still higher conversion of the starting material unsaturated aldehyde or still higher selectivity for an unsaturated carboxylic acid, is desired.
Patent Document 1: JP-A-8-206504
Patent Document 2: JP-B-4-79697
Patent Document 3: JP-A-2001-79408